Conventional Fiber Reinforced Polymers (FRP) offer very limited scope to the material designers in terms of desired electrical resistivities on their surfaces. While Glass FRP (GFRP) provide a highly insulating surface (surface resistivity>109 Ω/sq.), a Carbon FRP (CFRP) surface is a lot more conductive (surface resistivity≈100 Ω/sq.). However, the existing technologies do not allow these FRP materials to be further tailored for any custom specified intermediate surface resistivity value. The present technology helps to achieve the same in thermosetting polymer based composite materials with the conventional glass or carbon reinforcements wherein, the surface resistivity values can be tuned to any given order in the range of 109 Ω/sq. to 10−1 Ω/sq. based on the specific end use requirements.
Polymer composite materials often find themselves disadvantaged for a certain class of electrical/electromagnetic applications which demands specific ranges of electrical resistivities in the materials. In fact, if we place the entire gamut of materials alongside their respective surface resistivities, we get the resistivity ladder as shown in Table 1.
TABLE 1Resistivity LadderClass of MaterialsSurface Resistivity—Ω/sq.Polymers & Ceramics1012-1016Antistatic Materials 107-1011Statically Dissipative Composites101-106EMI Shielding Composites10−2-100 Carbon10−5-10−3Metals10−8-10−6
The extreme ends of the above spectrum are occupied by the base materials (metals/polymers/ceramics/carbon), and the intermediate ranges are mostly dominated by composite materials, coatings/paints etc. Numerous methods have been explored in the past to modify the electrical characteristics of otherwise insulating composites/paints etc. for certain specified applications (mostly antistatic & EMI shielding), including addition of special fillers like carbon black, carbon nanotubes (CNTs), nickel coated graphite (NCG), bromine intercalated graphite etc. For example, thermoplastic based antistatic polymers with chopped carbonaceous fibers which give a surface resistivity of the 104-1010 Ω/sq. order is known in the art (U.S. Pat. No. 5,820,788) as is a conductive laminate with room temperature resistance of 106-1012 Ω/sq. (U.S. Pat. No. 6,017,610). Even multilayer laminates with comparable surface properties are also reported (U.S. Pat. No. 6,740,410B2). Very recent literature also reveals CNT based antistatic coating compositions (U.S. Pat. No. 0,169,870A1). Various other polymer and resin compositions are also known which can be used for a variety of electrical/electromagnetic applications including antistatic surfaces and EMI shields. However, all these available technologies do not address the issue of tailoring conventional structural grade composite laminates for a wide enough range of surface resistivity properties (109-10−1 Ω/sq.) to render them versatile for most relevant applications. This is precisely the problem that the present disclosure intends to solve.
Prior art describes few methods of tailoring conventional polymer composites. In the prior art, tailoring polymer based materials for variety of electrical applications have always been guided by three major philosophies; viz. use of external/internal surfactants, use of conductive fillers and the use of polymeric additives. Each of these routes has their own sets of pros and cons. For example, U.S. Pat. No. 5,820,788 describes a class of materials with 8-20 wt. % of chopped carbon fibers in resin which results in surface resistivity values in the range of 104-1010 Ω/sq. Unlike the present invention, neither are such fiber filled resins capable of primary structural applications nor do they offer a wide enough scope of tailorability to the surface resistivity of the composites. The fiber loading also is very high, which is always associated with processing complications. Again, in U.S. Pat. No. 6,017,610, a conductive polymer (PAni) based laminate has been disclosed which has surface resistivity in the order of 106-1012 Ω/sq., but can be at best used as thermoplastic films and tapes. Similar observations can also be made for the materials disclosed in U.S. Pat. No. 6,740,410B2.
FIG. 1 illustrates the achievable range of surface electrical resistivities for various embodiments of the present invention in the form of surface resistivity vs. filler composition calibration curves. A brief comparison of the present invention vis-à-vis the prior art status can be understood from the illustration given in FIG. 2, wherein the prior art value ranges are superposed.